Ultra-relativistic electron radiation belt

September 24, 2012

The Proton/Electron Telescope (PET) aboard SAMPEX uses multiple energy-deposit and range measurements to identify electrons and light nuclides unambiguously over a wide energy range, which enables us to detect rare species even in regions of the magnetosphere with high fluxes of more abundant particles. In particular, the shock of 24 March 1991 injected a long-lasting “belt” of electrons with energies above 15 MeV at about L = 2 ((Blake et al., IEEE Trans. Nuc. Sci. 39, 1761 (1992); Vampola & Korth, GRL 19, 625 (1992); Blake et al., GRL 19, 821 (1992)), which was modeled theoretically by Li et al., GRL 20, 2423 (1993), and whose decay we were able to follow for more than a year after the July 1992 launch of SAMPEX (Looper et al., GRL 21, 2079 (1994)) despite the domination of that region of space by the high-energy protons of the inner zone.

We have begun to reanalyze the ultrarelativistic electron observations of PET over the entire SAMPEX mission, and in the course of this we have found another injection of a belt of ultrarelativistic electrons, attributable to the interplanetary shock event of 21 February 1994. This belt is weaker than that formed on 24 March 1991, but it is even more persistent, lasting to the present day. The accompanying figure displays these observations, as monthly averages over the radial (L) range from 1 to 6 Earth radii of the countrate of “RNG” electrons: this subset of the PET data has a complicated energy response function, as shown in Looper et al. (1994), which peaks at about 8 to 15 MeV with a geometry factor of about 0.1 cm^2 sr.

In the figure to the left, the remnants of the 24 March 1991 electrons are clearly visible around L = 2, strong at the start of the mission but decaying with time. There are several artifacts of spacecraft attitude and instrument mode changes visible here; most noticeable is the apparent three-month pulsation of this belt because of variation of observed pitch angles as the SAMPEX orbit precesses with a six-month period. From about October 1993 to February 1994 the PET instrument was in a mode that resulted in significant background in this rate in regions of high particle flux (notable are the spurious proton response below about L = 1.5 and the response to lower-energy relativistic electrons from about L = 4 to 5.5). Despite this background, it is clear that fluxes of ultrarelativistic electrons rose over a broad range of L, from about 2.5 to 5, in February 1994. Inspection of the daily- averaged data shows that electrons in this energy range were present during the very strong outer-zone enhancement, driven by high-speed solar wind, that began on 9 February 1994, and we suspect that the new belt owes its origin to the fact that the interplanetary shock struck at the end of this enhanced period.

These electrons can be seen to form a belt that persists and migrates inward for years afterward, while the 24 March 1991 belt disappears at about the time of formation of the new belt. Various other artifacts modify the appearance of the data over time; the apparent sudden increase in flux at around L = 2.5 in May 1994 is due to the increased observations of higher fluxes (already present) near 90 degrees pitch angle as the result of a change in the spacecraft pointing algorithm, and the resumption of three-month “pulsing” from May 1996 though April 1998 is because the spacecraft was spinning at 1 RPM during most of these two years, which again caused a variation of the average pitch angle observed as the spacecraft orbit precessed. Nonetheless, the new belt can clearly be traced to the end of the plot in November 1998, as it penetrates inward over time even deeper than did the 24 March 1991 belt, with the peak eventually reaching about L = 1.3.

Also notable in this plot is the absence of any strong outer-zone activity at these high energies, even during periods when lower-energy relativistic electron fluxes were elevated by many orders of magnitude over quiet-time values. (As noted above, the increase in countrate at L= 4 to 5.5 from October 1993 to February 1994 is mostly spurious; also, the increase across all L-values during the time when the spacecraft was spinning at 1 RPM is attributable to improved observations in this period of atmospheric albedo particles, as discussed in this space about a year ago.) Some electrons in this energy range are visible in the daily-averaged data during periods of outer-zone enhancement, but most of the ultrarelativistic electrons we have observed are seen as part of these persistent shock-injected belts. We are continuing to reanalyze all the PET electron data, and hope to extend observations of this new belt to lower energies.